Thanks, Michael.
Ok, I understand most of what you did, but ...
do nums <- askForNumbers; map sqrt nums
ERROR - Type error in final generator
*** Term : map sqrt nums
*** Type : [Integer]
*** Does not match : IO a
do nums <- askForNumbers; printList nums -- works fine
Thanks,
Jeff
-------------------------------------------------------------
Hello,
I have started reading "Yet Another Haskell Tutorial" by Hal Daum´e III which can be found here
http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf
One of the early examples in section 3.8 pg. 35
is this
askForWords = do
putStrLn "Please enter a word:"
word <- getLine
if word == ""
then return []
else do
rest <- askForWords
return (word : rest)
I want to print the returned list and everything I try fails.
I have tried the following:
printList l =
if length l >= 1
then do putStrLn (head l)
printList (tail l)
else putStrLn("")
f = printList askForWords
and I get
Expression : printList askForWords
*** Term : askForWords
*** Type : IO [[Char]]
*** Does not match : [[Char]]
I believe one of the following will work for you:
f = askForWords >>= printList
f = do
words <- askForWords
printList words
*************************************
The exercise right below this asks for a very slight modification to read numbers instead.
However, I am confused about how to convert strings to numbers.
If I type in the hugs interactive console
read "5" + 3 --> 8 -- ok perfect
However
read "5" gives
ERROR - Unresolved overloading
*** Type : Read a => a
*** Expression : read "5"
Yet page 33 of the tutorial has the following code:
doGuessing num = do
putStrLn "Enter your guess:"
guess <- getLine
let guessNum = read guess -- ok in let stmt, but not at repl prompt?
The problem here is type inference. The statement read "5" has type "(Read a) => a", which basically means anything that implements the class "Read." When you do read "5" + 3, the read "5" gets the type of the 3. I assume that in the latter case, you use the expression guessNum in a way later on that the compiler can infer its type.
Anyway I take the info that has been presented and create this function:
askForNumbers = do
hSetBuffering stdin LineBuffering
putStrLn "Give me a number (or 0 to stop)"
numStr <- getLine
let num = read numStr
if num == 0
then return []
else do
rest <- askForNumbers
return (num : rest)
However, when I try to use it, like say
map sqrt askForNumbers
ERROR - Type error in application
*** Expression : map sqrt askForNumbers
*** Term : askForNumbers
*** Type : IO [Integer]
*** Does not match : [a]
Similar to above, try this:
do nums <- askForNumbers
map sqrt nums
*********************************************************
Is there a way to write printList to handle Strings or numbers?
Or should I write
printList (map show askForNumbers)
Note: you should probably do this using mapM_, but for simplicity, I'll do it using explicit recursion:
printList [] = putStrLn "" -- or return () if you don't want the extra blank line
printList (x:xs) = do putStrLn (show x)
printList xs
If you have any questions about how these worked, let me know!
Michael